Optical interferometers are often used to make accurate measurements of position. For example, in semiconductor fabrication applications, a movable stage having a wafer mounted thereon must be accurately positioned with respect to an optical system that projects a pattern image onto the surface of a wafer. Through photolithography, the pattern image defines the patterning of a constituent layer of a semiconductor device formed in a small region of the wafer. The pattern image has to be aligned very accurately with elements of the semiconductor device already formed in or on the wafer. The accuracy required of the positioning operation depends on the feature size of the pattern image. Decreasing feature sizes have driven a demand for ever more accurate positioning mechanisms, and hence, metrology systems for accurately measuring the position of the stage.
Interferometer-based position metrology systems are typically used to measure position. Interferometers typically measure displacement, i.e., a change of position. To obtain a measurement of position from a metrology system that measures displacement, the stage is initialized to an accurately-known start position and is then moved to a current position. The metrology system measures the displacement of the current position from the start position, and adds the measured displacement to the start position to obtain the current position of the stage. When the stage makes more than one movement to reach the current position, the measured displacements of all the movements are summed and the result is added to the start position to obtain the current position.
Recently, interferometer-based metrology systems of the types disclosed by Trutna, Jr. et al. in United States patent application publication no. 2007/0146722 and by Schluchter et al. in U.S. patent application Ser. Nos. 11/686,855 and 12/172,810 have been used to measure the displacements of a movable object such as a stage. All of these disclosures are assigned to the assignee of this disclosure and are incorporated herein by reference. Such metrology systems have an approximately constant optical path length between an interferometer head and the stage, and are therefore less susceptible to errors due to variations in the refractive index of air caused by variations in temperature, humidity, etc.
Although differing in details, in the above-mentioned interferometer-based metrology systems, an interferometer head directs a measurement beam of light onto a diffraction grating typically mounted on the underside of the movable stage. The measurement beam is incident on the diffraction grating at a non-zero angle of incidence. In the direction or directions in which the stage is capable of large-scale movement, the diffraction grating has a dimension greater than the maximum range of movement of the stage in that direction, and has lines extending in directions which are not parallel to each direction of movement. The diffraction grating diffracts the measurement beam back to the interferometer head. In the metrology system disclosed by Trutna, Jr. et al, the interferometer head combines the measurement beam with a reference beam reflected by a reference mirror whose position does not change. In the metrology systems disclosed by Schluchter et al, the interferometer head combines a first sub-beam derived by the diffraction grating diffracting the measurement beam at one order with a second sub-beam derived by the diffraction grating the measurement beam at another, different, order or derived by the diffraction grating diffracting another measurement beam at the same or a different order. In both metrology systems, the interference fringes that occur in the combined beam as the stage moves are counted to provide a fringe count and the fringe count is processed to provide a measure of the displacement of the stage. Optionally, the measured displacement is added to the start position of the stage to obtain a measure of the current position of the stage.
As noted above, the stage is capable of large-scale movement in a first direction. In applications such as the above-mentioned semiconductor device photolithography application, the stage is additionally capable of small-scale movement in a second direction. The second direction is typically orthogonal to the first direction and to the plane of the grating. References in this disclosure to displacement are to be taken to refer principally to the first direction. Measurement of the position or the displacement of the stage is principally of interest in the first direction. References in this disclosure to separation are to be taken to refer principally to the second direction.
In some applications, the stage is additionally capable of large-scale movement in a third direction typically mutually orthogonal to the first direction and the second direction. The first and third directions are therefore nominally coplanar with the grating. With respect to such applications, references in this disclosure to displacement are to be taken to refer principally to either or both of the first direction and the third direction. Measurement of the position or the displacement of the stage is principally of interest in the first direction and the third direction in such applications.
Imperfections in the diffraction grating impair the accuracy of the displacement measurements obtained simply by counting the fringes. The displacement measurement obtained from the fringe count does not intrinsically have the accuracy required for such applications as semiconductor device photolithography. To reach the required accuracy, the metrology systems have to be calibrated. The metrology systems are sufficiently precise that, after they are calibrated, they can consistently provide the accuracy needed for such applications as semiconductor device photolithography with feature sizes as small as a few tens of nanometers.
The metrology system is calibrated by mounting a special calibration wafer on the stage. The calibration wafer has an array of fiducial marks arrayed on its major surface at accurately-known displacements from one another. Calibration involves moving the stage to precisely align each of the fiducial marks with the optical system of the host photolithography apparatus and measuring the displacement of the stage using the metrology system. Differencing the known displacement and the measurement displacement yields a calibration value pertaining to the known displacement. The need to precisely align each of a large number of fiducial marks with the optical system during the calibration process makes the calibration process very time consuming. Time spent calibrating the metrology system is time in which the photolithography apparatus cannot be used to process production wafers.
Additionally, when the stage is additionally capable of small-scale movement in the second direction, since the calibration values depend not only on the displacement in the first direction but also on separation in the second direction, the above-described calibration process has to be repeated at a number of different separations between the stage and the interferometer head. The need to perform the calibration process several times to complete the calibration of the metrology system multiplies the time needed to calibrate a conventional metrology system.
What is needed, therefore, is a metrology method and system having a substantially reduced calibration time.